This invention relates generally to a refrigerant flow control device in a closed cycle refrigeration system, and in particular to a flow controller or throttle device that provides rapid cool down and low vibration steady-state performance.
Every throttle cycle refrigerator incorporates a throttling device as a means for reducing the refrigerant pressure from the compressor discharge before the refrigerant flow enters the evaporator. However, accomplishing this pressure change is complicated by the fact that operating conditions are significantly different at start up of a warm system as compared to steady-state operation at desired loads and operating temperatures. For good overall performance, the throttling device should provide the desired vapor pressure in the evaporator under steady-state conditions and also during cool down.
For fast cool down, it is necessary that the refrigerant flow rate be higher than at steady state operating conditions. Thus at cool down, it is desirable that the return pressure to the compressor be high enough to provide sufficient flow rate through the system; compressor pumping capacity increases as the inlet refrigerant pressure to the compressor increases. Unfortunately, a fixed restrictor such as an orifice or capillary tube limits the compressor performance and refrigerant flow rate most just when maximum flow rate would be beneficial. This occurs because at start up the restrictor flows a refrigerant of low average density, that is more gaseous than liquid. Thus, the simplest means used for reducing the pressure of the refrigerant flow, that is, a fixed geometry restrictor such as a capillary tube or small orifice, which are simple in design and easy to manufacture, have low mass flow rates at start up and slow cool down if sized for subsequent steady-state performance.
To improve the cool down characteristic and thereby reduce the cool down time, many constructions of throttling devices have been proposed. For example there are U.S. Pat. Nos. 3,320,755; 3,257,823; 3,457,730, and British patents Nos. 1,164,276 and 1,297,132. Japanese patent No. 24498 (1970) is also of interest. These patents disclose various means for increasing the orifice size in the throttle device during cool down. This increase in orifice size is achieved, for instance, using a needle that is actuated by a gas filled bellows, or by using the difference in thermal expansion of different materials to move the needle relative to an orifice. These throttling devices incorporate many high precision parts, which result in high cost to manufacture and low reliability in performance of the whole device.
Further, in many applications of closed cycle refrigeration systems, vibration is a critical parameter to be controlled. The energy dissipated when refrigerant flow is throttled from high pressure to low pressure, as is required in a closed refrigeration cycle, is a major contributor to vibrations at the cold end of the system. Whereas the vibration induced by the throttle device may be subsequently damped and isolated by special constructions, which add to the complexity of the system at the cold end, elimination of the vibrations where they originate in the throttle device would be advantageous.
Preliminary investigations indicate that vibration as measured at the load interface for the closed cycle refrigeration system has a steady state component and a pulsating component when the system uses a capillary tube throttle. A multi-phase refrigerant, which was used in the investigations, having two liquid phases and one vapor phase flowing through the capillary throttle valve, produced an unacceptable pulsating flow due to the large density change as the refrigerant pressure drops from high pressure compressor discharge to low pressure at the cooler evaporator and the compressor inlet. Smaller diameter capillary tubes increase the refrigerant velocity in the tube for a given mass flow rate of refrigerant. It has been found that this high velocity tends to increase the steady state portion of the throttling vibration.
What is needed is a throttle device for a closed cycle refrigeration system that is simple in construction, provides rapid cool down and produces low vibration at the cold end especially at steady-state conditions.